In many air duct systems, particularly those involving variable air volume, it is desirable to be able to measure the air flow in the duct work. A difficult problem is encountered because of the non-uniform distribution of air flow in ducts caused by wall friction, duct length, elbows, stratification, obstructions and the like so that even under laboratory conditions, the measurement of actual flow is often tedious and cumbersome.
In the prior art there have been two measurement techniques that operate reasonably well to determine the actual flow. The first of these techniques involves the pressure drop across a calibrated orifice or baffle and the output is a reasonably reliable measure of total flow. However, in order to get good readings, the pressure drop has to be fairly large because low pressure transducers are expensive and hard to work with. The large pressure drops represent loss of energy which make this method of measuring flow an expensive undertaking. While in the laboratory this may be acceptable, it is not suitable for field applications.
The second accepted measurement technique is a "Pitot tube traverse". In this technique, a single Pitot tube is sequentially positioned at various points in the duct work to obtain the velocity pressures at these points. The square root of the velocity pressures are then averaged to obtain the average velocity and consequentially the average flow. Obviously the moving of a single Pitot tube to different positions is a slow and tedious procedure that is only fit for laboratory work but the method is one which produces very little disruption of the flow and incurs a minimum pressure drop. An extension of this technique has been used by some companies who have developed "flow stations" incorporating permanently mounted arrays of Pitot tubes so as to avoid the manual positioning of a single. These systems pneumatically average the Pitot tube pressures to arrive at an average velocity pressure signal and the square root of this average is then taken which produces an output that is not as accurate as the traverse since the square root of the average is taken rather than the average of the square roots. Nevertheless, a very good approximation of the true flow is obtained with the resulting error being probably about three percent. The Pitot tube flow station has become an industry standard although it still has some problems because the flow station must be fabricated to match particular duct sizes and since the relationship between flow and velocity pressure is not linear it is necessary to use non-linear gauges or square root extractors on the output pressure causing the system to be relatively expensive.